You would need to take the 2.5mm² spur from the socket to a FUSED CONNECTION UNIT, and then from there to the switch in 1.0mm² or 1.5mm².
As your current socket is already a spur, you will need to take your spur for the FCU from a different socket, or make the existing socket part of the ring. This assumes your sockets are wired on a ring circuit. Do a search for ring circuits or socket circuits.
Example of an FCU:
